Chemistry · Lesson

Oxidation states, oxidation and reduction

Assign oxidation states without treating them as every atom’s actual charge. Identify oxidation, reduction and the agents that undergo the opposite named change.

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By the end, you can

Check oxidation-state sums against the whole-species charge.
Distinguish oxidation state, formal charge and partial charge.
Identify oxidizing/reducing agents and a disproportionation example.

Oxidation state is a defined electron-assignment model

An oxidation state assigns an atom a formal charge after an ionic approximation of bonding: heteronuclear bonding electrons are assigned according to the applicable electronegativity convention, while homonuclear bonds divide them equally. Introductory rules summarize common simple cases.

A monatomic ion’s oxidation state equals its ionic charge. In a covalent molecule, an oxidation-state assignment is different from formal charge or the atom’s actual partial charge. Assigning H +1 and O −2 in neutral water does not make water a collection of free H⁺ and O²⁻ ions.

The sum must reproduce the species charge

For nitrate NO₃⁻, using O −2 gives x + 3(−2) = −1, so N has oxidation state +5. The whole nitrate ion still has charge −1. Keep the atom assignment and the species charge separate.

Usual H +1 and O −2 assignments have exceptions. Oxygen is −1 in H₂O₂, and hydrogen is −1 in NaH when Na is +1. Superoxides and oxygen fluorides require other assignments. A mixed-valence average need not describe the local state of every atom individually.

Keep atom assignments separate from whole-species charge

Species	Specified assignments	Sum	Whole-species charge
NO₃⁻	N +5; each O −2	+5 − 6 = −1	−1
H₂O	Each H +1; O −2	+2 − 2 = 0	0
H₂O₂	Each H +1; each O −1	+2 − 2 = 0	0
NaH	Na +1; H −1	+1 − 1 = 0	0

Introductory oxidation-state bookkeeping for the specified species. Values are formal assignments and sum to the overall charge; they are not measured atomic partial charges or a statement that neutral covalent molecules contain free constituent ions.

Track which species changes

Oxidation increases oxidation state; reduction decreases it. In a simple electron half-reaction, Fe²⁺ → Fe³⁺ + e⁻ is oxidation, while Cu²⁺ + 2e⁻ → Cu is reduction. The electron is on the side required to balance the species charges.

An oxidizing agent brings about oxidation of its partner while itself being reduced. A reducing agent brings about reduction while itself being oxidized. In covalent redox chemistry, the formal bookkeeping need not imply a separate literal electron travelling from one isolated ion to another.

One species can undergo both directions of change

For the balanced equation 2 H₂O₂ → 2 H₂O + O₂, oxygen begins at −1. Oxygen ending in water is −2; oxygen ending in O₂ is 0. Thus some oxygen is reduced and some oxidized: this is disproportionation.

Both sides contain four H and four O atoms, with no net charge. The overall equation contains no leftover electrons. It records net composition and oxidation-state changes, rather than establishing a reaction mechanism, rate or catalyst.

Worked examples

Find nitrogen’s nitrate oxidation state

For NO₃⁻, use the ordinary oxygen assignment −2. Find nitrogen’s oxidation state and distinguish it from the ion’s charge.

Let nitrogen’s oxidation state be x.
The three oxygens contribute −6.
Set the sum equal to the species charge: x − 6 = −1.
Solve x = +5.

Nitrogen is assigned +5; the whole nitrate ion has charge −1. These describe different quantities.

Try it yourself

Does an oxidation state always equal an atom’s actual partial charge?
Show the answer and reasoning
No.
It is a formal ionic-approximation assignment; partial charge describes a different quantity.
What must the oxidation-state sum in NO₃⁻ equal?
Show the answer and reasoning
−1.
It equals the charge of the whole nitrate species.
Does the oxidizing agent itself undergo oxidation or reduction?
Show the answer and reasoning
Reduction.
It undergoes reduction while its partner undergoes oxidation.
What happens to oxygen’s −1 assignment in the stated peroxide disproportionation?
Show the answer and reasoning
Some becomes −2 in water and some becomes 0 in O₂.
The same starting species supplies reduced and oxidized products.

What to carry forward

Sum oxidation states to the whole-species charge.
Distinguish formal assignments from formal and partial charges.
An oxidizing agent is reduced; a reducing agent is oxidized.
Net redox bookkeeping does not by itself give a mechanism.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.